1. Field of the Invention
This invention relates to a photoactive material making use of photochromism. More particularly it relates to a photochromic compound expressed by the formula (I) or its precursor expressed by the formula (II), the respective formulas being described below
2. Description of the Related Art
In recent years, with rapid development of information industries, high density memory media have been required. Optical data storage media have a possibility of remarkably surpassing magnetic data storage media employed so far, in the aspect of memory density; hence a number of researches have been carried out. Optical media include that of Write Once type wherein a recording is possible only once and that of Rewritable type wherein reversible recordings are possible, and photochromic compounds belong to materials employed as the latter Rewritable type optical media.
Photochromic compounds refer to a compound which changes the spectrum by irradiating a light of a certain wavelength and returns to the original spectra by heat or irradiation of a light of another wavelength. When such photochromic compounds are intended to be applied to optical memory media, a photochromic compound which does not fade thermally at room temperature is required for an archival storage. An example of such a compound is disclosed in M. Irie, Jpn. J. Appl. Phys., 28 supple 28-3, 215 (1989).
However, as to such a photochromic compound,since the molecules are isomerized in proportion to the number of photons absorbed in principle, the media employing the compound have a drawback that even a very weak light employed for reading erases the memory gradually.
Further, photochromic compounds are characterized in that they change the colors by photoirradiation; thus it has been proposed that the compounds can be utilized for display elements. In this case, too, a drawback has been raised that the display fades out in ordinary sunlight or electric light.
As a method for preventing such a record and display from breaking, several methods have been proposed. A method is to use thermal threshold in the photochromic reaction of J-aggregates of spirobenzopyrans. The photochromic reactions depend on the laser light intensity. When the light intensity is high enough to heat the media above the threshold temperature, fading of color proceeds, while the fading is suppressed with a weak laser light. Non-destructive read-out more than 1,000 times was achieved by using the method (J. Hibino, Basic Technologies for Future Industries, the 3rd Symposium on Photonic Materials, Extended Abstracts, p165(1992)). Another method is to read the memory by two lasers with different wavelengths, in which diarylethenes with gated photochemical reactivity were used as the media (M. Irie et al., Optical Memory Symposium'92 (1992)). Physical property changes of liquid crystals by the photochromic reactions were also used as the stable memory, which can be read without destruction with the laser light which can not induce the photochromic reactions (K. Ichimura, Kagaku to Kogyo, 43,783(1992)). All of these methods, however, have drawbacks, such that the heating causes the degradation of the media and they need more than three lasers.
It is known that the efficiency of the photoisomerization is light-intensity dependent (J. Chem. Soc. Perkin Trans 2, 873, 1991), but according to the example, even when the light intensity is very weak, the photoreaction proceeds to some extent; hence it is impossible to completely overcome non-destructive read-out. Further, since another isomer is unstable, it is difficult to store the memory and display for a long time, and change in the absorption spectra are poor.
The present inventors have made extensive research to find out a photochromic compound which is possible to solve the above-mentioned problems. As a result, we have found a photochromic compound which does not isomerize when the light intensity is weak, but isomerizes only when the intensity is strong, and thus have completed the present invention.
As apparent from the foregoing, the object of the present invention is to provide a photochromic compound that recordings do not fade out in the reading process and are thermally stable, and a photochromic medium using the compound.